Giffords is accusing senators who opposed new gun regulations of "cowardice," saying their decisions are "based on political fear and on cold calculations about the money of special interests like the National Rifle Association."

The former Arizona congresswoman's remarks were published on The New York Times' op-ed page Wednesday. She has become a vocal gun control supporter since she was shot in the head at a rally near Tucson two years ago.

The proposal to expand background checks fell short of the 60 votes needed to advance in the Senate. An attempt to ban assault-style rifles failed as well, along with a ban on high-capacity ammunition magazines.